A light shade of a colour, like blue, across the whole background of a form relieves the eye from the harshness of a stark white background. And if the answer spaces people have to fill in are white, the tinted background both cues them in to where they write, and allows them to visually sense how much they have to fill in.

In paper forms the specific sub-questions cannot be hidden from the user when they are not relevant to them - but in interactive forms this is relatively easy to do. Use progressive revealing as much as possible in your form design to shield your users from questions they do not need to see.